This invention relates to a method for treating cardiac arrhythmia in mammals by administering a 4-fluoro-.gamma.-(4-alkylpiperidino)-butyrophenone or a 4-fluoro-.gamma.-(4,4-dialkylpiperidino)-butyrophenone or an acid-addition salt thereof. Heretofore a great number of different compounds have been investigated in an effort to develop effective drugs for therapeutic treatment or cardiac arrhythmia (hereinafter referred to as arrhythmia). In animal tests many of these compounds have shown antiarrhythmic effect, but have not been acceptable for clinical use, because of low activity and/or side effects. Pharmacologically the carbamoyloxy decahydroquinoline derivative L 7810 (4-carbamoyloxy-1-(4-fluorophenyl)-4-oxobutyl)decahydroquinoline) has been reported (Bagwell et al., Br. J. Pharmacol. 1973:48:183- 93) to exert antiarrhythmic properties. By way of example, the prior art has employed for therapeutic treatment lidocaine, procainamide, quinidine, propranolol, etc., compounds having activity as antiarrhythmic agents, but at the same time producing various toxic effects.